Species and Taxonomy

There are millions of species on Earth, so how do scientists decide whether two organisms belong to the same species, and how are the different species organised? This page covers what a species is, why courtship matters before mating, and how species are classified and named.

Defining a Species

For example, horses breed with horses and produce fertile offspring. However, if two different species interbreed (e.g. a horse and a donkey produce a mule), the offspring are sterile and cannot reproduce.

Courtship Behaviour

Courtship behaviour is a set of actions performed by animals before mating (for humans, this ‘set of actions’ could be described as similar to dating).

It is a necessary precursor to successful mating, and without it mating is unlikely to occur.

Role of Courtship in Species Recognition

Courtship signals allow individuals to identify members of their own species. Only individuals that produce and respond to the correct signals will mate.

This prevents interbreeding between species.

Courtship also helps an animal:

• Find a mate that is sexually mature (able to breed)
• Synchronise mating, so it happens when the female is most likely to be fertile
• Form a pair bond, which in some species helps the parents raise the young together

Examples of courtship behaviour:

• Peacock (male) performs elaborate rituals and dances for the female (called a peahen), fanning out vibrant feathers.
• Frogs use special advertisement calls to attract female frogs.

Phylogenetic Classification

How are species organised?

Phylogenetic classification organises species according to their evolutionary origins and relationships.

Species that share a more recent common ancestor are grouped more closely together.

Key features of the system:

• Arranged as a hierarchy.
  • Groups are nested within larger groups
• No overlap between groups.
  • Each organism belongs to exactly one group at each level
• Each group is called a taxon (plural: taxa)
  • See these on the diagram below

Exam questions often ask you to interpret phylogenetic trees. Try these questions below:

Using the diagram above, which organism diverged first from the common ancestor (0)?

Organism 5. Its branch splits off at the first branch point after the common ancestor (0), before any other branching happens. This means 5 diverged earliest and is the most distantly related to all the others.

Follow the branch points from left to right, like a timeline: the further left a split is, the longer ago it happened.

The Hierarchy

Phylogenetic classification tells you why organisms are grouped together, because they share a common evolutionary ancestor. The hierarchy of taxa tells you how groups are arranged.

From broadest to most specific:

You don’t need to learn these example names: they just show how each level works, using humans. What you need to know is the order of the levels.

Mnemonic: Dear King Philip Came Over For Good Soup

Binomial Nomenclature

Every species is given a unique two-part Latin name:

• First name: the genus (capitalised, e.g. Homo)
• Second name: the species (lower case, e.g. sapiens)

Written in italics (or underlined if handwritten): Homo sapiens

This binomial system is used internationally, so scientists in any country refer to the same organism by the same name, regardless of local common names/differences.

Investigating Evolutionary Relationships

Traditional classification relied on comparing observable (phenotypic) characteristics. This is less accurate because:

• Unrelated species can look similar, and closely related species can look very different
• Characteristics are affected by the environment as well as genes (e.g. height, weight)
• It’s subjective, so scientists can disagree about which features matter most

Modern techniques allow more accurate comparison by looking directly at the molecules:

Molecular Comparisons

• DNA base sequence: comparing the base sequences of genes across species
• mRNA base sequence: comparing base sequences of mRNA produced by specific genes
• Amino acid sequence of proteins: comparing the primary structure of proteins coded for by shared genes

Species with more similar sequences share a more recent common ancestor and are more closely related.

Immunological Comparisons

If a protein from one species is put into a different species, the protein acts as an antigen, because the second species’ immune system sees it as foreign. That species then produces antibodies against the protein, which are complementary in shape to it.

Let’s look at an example:

1. A protein from human blood (species 1) is injected into a rabbit (species 2), and the rabbit makes antibodies against it
2. These antibodies are collected and mixed with the same protein from other species (e.g. a chimpanzee (species 3) and a dog (species 4))
3. If the other species’ protein has a similar shape, the antibodies bind to it and form a precipitate (clumps)

The more precipitate forms, the more similar the protein is to the human one, so the more closely related that species is to humans. The chimpanzee’s protein would give lots of precipitate; the dog’s would give much less.

Exam Question Practice